A conventional intake manifold for preventing a reduction in engine output in an engine provided with a plurality of intake tubes that extend at different slope angles from a surge tank is disclosed in JP 10-184469 A.
The manifold disclosed in JP 10-184469 A has a single surge tank and four vertically arranged intake tubes that are in communication with the surge tank. With an intake manifold disposed on one side of the engine, the slope angles of the intake tubes connected to the four combustion chambers formed in the cylinder head are different. Therefore, to prevent the length of the intake tubes from being nonuniform, the intake tube having the lowest slope angle is disposed so as to extend most greatly to the side, and the intake tube having the highest slope angle is disposed so as to extend by the least amount to the side, thus compensating for the nonuniform lengths of the intake tubes and achieving a substantially fixed intake tube length.
In the above-described intake tubes, each intake tube has the same circular cross-sectional shape.
When the cross-sectional area of the tubes is increased to achieve higher engine output, the spacing between the openings to the cylinders becomes narrow, and sufficient output cannot be achieved because of intake interference when the ignition sequence begins from the intake tube in the highest position and ends with the intake tube in the lowest position.
The dimension in the height of the engine increases when the spacing between cylinders is increased in order avoid intake interference, leading to an increase in the size of the intake manifold composed the intake tubes.
When the intake tubes are given a large cross-sectional area in order to increase engine output, the cross-sectional shapes of the intake tubes are increased in the longitudinal and transverse directions. In other words, in the intake manifold on one side of the cylinder block of an engine, the size of the cross-sectional shape in the height direction of the vertically arranged intake tubes is increased, or the size of the cross-sectional shape of the intake tubes is increased in the width direction from the intake tube in the highest position and to the intake tube in the lowest position.
Therefore, the size of the intake manifold disposed on one side of the engine is increased in the height direction, the degree of freedom in the layout of the equipment disposed in the upper portion of the engine is reduced, the entire height of the engine cover is increased, or the size in the width direction is increased beginning with intake tube in the highest position, and the width of the engine cover is increased from the lower portion to the upper portion. It is therefore impossible to adopt an engine cover in which the upper portion of the cover is made narrow and the cover gradually becomes wider in the downward direction, and other problems are encountered.
In view of the above, there is a need for an outboard engine unit in which the dimension in the height of vertically arranged intake tubes is minimized, the undesirable effects of intake interference between cylinders is reduced, and engine output is improved for an arrangement of intake tubes in the intake manifold of an engine.